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Date:   Wed, 31 Mar 2021 22:32:21 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Raj Ashok <ashok.raj@intel.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/1] x86/tdx: Handle MWAIT, MONITOR and WBINVD
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On Wed, Mar 31, 2021, Kuppuswamy, Sathyanarayanan wrote:
> 
> On 3/31/21 3:11 PM, Dave Hansen wrote:
> > On 3/31/21 3:06 PM, Sean Christopherson wrote:
> > > I've no objection to a nice message in the #VE handler.  What I'm objecting to
> > > is sanity checking the CPUID model provided by the TDX module.  If we don't
> > > trust the TDX module to honor the spec, then there are a huge pile of things
> > > that are far higher priority than MONITOR/MWAIT.
> > 
> > In other words:  Don't muck with CPUID or the X86_FEATURE at all.  Don't
> > check it to comply with the spec.  If something doesn't comply, we'll
> > get a #VE at *SOME* point.  We don't need to do belt-and-suspenders
> > programming here.
> > 
> > That sounds sane to me.
> But I think there are cases (like MCE) where SEAM does not disable them because
> there will be future support for it. We should at-least suppress such features
> in kernel.

MCE is a terrible example, because the TDX behavior for MCE is terrible.
Enumerating MCE as supported but injecting a #GP if the guest attempts to set
CR4.MCE=1 is awful.  I'm all for treating that as a one-off case, with a very
derogatory comment :-)